Split parabolic radar antenna utilizing means to discriminate against crosspolarized energy



.SEARCH RC June 4, 1963 J. G. MocANN ETAL 3,092,834

SPLIT FARABOLIC RADAR ANTENNA UTILIZING MEANS T0 DISCRIMINATE AGAINSTCROSS-POLARIZED ENERGY Filed Deo. 23, 1958 2 Sheets-Sheet 1 0 4 2 JW 5 22 www. 2

June 4, 1963 J. G. MGCANN ETAL 3,092,834

SPLIT PARABOLIC RADAR ANTENNA UTILIZING MEANS TO DISCRIMINATE AGAINSTcRoss-PoLARzED ENERGY Filed DGO. 23, 1958 2 Sheets-Sheet 2 MG. zz.

INVENToRs Jog 9. Mcm/VN United States Patent Oil ice 3,092,834 PatentedJune 4, 1963 SPLH PARABOLEC RADAR ANTENNA UTILIZHNG MEANS T DISCRIMINATEAGAI YST CROSS- POLARIZED ENERGY Joe G. McCann, Encino, Robert I.Stegen, Van Nuys, and William llalstrom, Encino, Calif., assignors, bymesne assignments, to Canoga Electronics Corporation, Van Nuys, Calif.,a corporation of Nevada Filed Dec. 23, 1958, Ser. No. 782,599 Claims.(Cl. 343-756) This invention relates to airborne radars and, moreparticularly, to an antenna system for a somewhat unsymmetricalgenerally dish-shaped or parabolic reflector used in radar trackingsystems.

In the past, it has been the practice to use a dish-shaped reflector orradar dish split in half by a plane through its symmetrical axis. Thehalves are generally kept together for searching operation; however, ithas been the practice to rotate these halves through a small angle abouta line tangent to the symmetrical center of the parabolic surface in theplane of the split away from each other to a position in which they aremaintained stationary with respect to each other for trackingoperations. In this case, the halves are illuminated withelectromagnetic energy having a plane of polarization or E-plane throughthe symmetrical axis of the antenna system parallel to the split.

During tracking, both halves are rotated in a conventional manneralthough their symmetrical axes are maintained at a lxed angle withrespect to each other. The split arrangement is employed to broaden themain lobe of the antenna in a plane perpendicular to the E-plane, i.e.,the H-plane.

To the present time, a serious disadvantage has accompanied the use ofthe above-described split reflector during tracking. As is well known,an approximately sinusoidal modulation envelope is normally imposed onreceived electromagnetic energy by reflection from ay target which isspaced from the axis of rotation during rota-tion in tracking. Theangular position of the target relative to the radar transmitter is thusconventionally determined for tracking or other purposes by measuringthe magnitude of this modulation and comparing its phase with analternating analog signal representative of the instantaneous rotatingposition of the reflector. The magnitude of the modulation envelope,thus, is a function of the angle between the rotational axis and a linethrough the target and through the point of radiation, and the phase ofthe modulating envelope is a function of the angular position of a planethrough the same line and the axis of rotation and another fixedreference plane through the axis of rotation.

The parabolic split radar dish operates satisfactorily generally when atarget exists at a relatively large angle from the axis of rotation, buta phase shift of the modulating envelope occurs at relatively smallangles which introduces error. IIn fact, at a certain small angle for asplit radar dish of a given geometry, the phase shift error appears tobe approximately 90 electrical degrees. The split radar dish thus hasbecome practically useless for accurately tracking targets located closeto the axis of rotation of the dish or close to boresight.

The present invention overcomes the above-described and otherdisadvantages of the prior art by providing in combination therewith,means associated with a split radar dish to discriminate againstcross-polarized electromagnetic energy. A deiinition of cross-polarizedelectromagnetic energy will be given hereinafter. However, even thegeneral manner in which the invention performs its desired function,i.e., the manner in which it eliminates the above-described phase shifterror in the modulation envelope produced by off-boresight targetsduring tracking, cannot be understood without resorting to at least somebasic antenna theory.

Firstly, portions of the prior art parabolic reilector may be defined asbeing made up of four identical quadrants divided by perpendicular E-and H-planes through the symmetrical axis thereof. When a plane wave ofa given polarization is intercepted by a quadrant, currents flow inplanes parallel to both of the E- and H-planes. This may be understoodby recognizing that the interception of a plane through a lineperpendicular to the axis of symmetry of a parabola of revolution,intercepting the axis at the point of feed or illumination, and theparabola itself delines a predetermined curve. The projection of thiscurve on a plane perpendicular to the E- and H-planes is then circular.Orthogonal vector currents combining to make for circular current tlowthus explain why currents actually do flow in planes parallel to theH-plane. Electromagnetic energy transmission caused by current flowingin planes parallel to the H- plane on the radar dish conductive surfaceis called cross-polarized. This phenomenon is not unknown in the priorart and is explained in vol. l2, Radiation Laboratory Series, page 419.Reflection from a radar-dish of cross-polarized energy exists in theabove-mentioned four quadrants but radiation from adjacent quadrants areout of phase with respect to each other although, in phase transmission,the energy at 'the common point of feed from which they were derived wasin-phase energy. Thus, to the present time, cross-polarized lobes werethought unimportant, firstly, because it was thought they would canceleach other out to a certain extent and, secondly, because they wouldsimply add a second harmonic signal to the fundamental frequency ofsignal of the modulating envelope produced by an oE-boresight targetduring the antenna rotation of tracking. In the prior art, the secondharmonic signal was theorized because antenna rotation would carry thetwo nearest-totarget cross-polarized lobes alternately nearer andfarther away from a target while the main lobe would be carried towardand away from the target only once during a single cycle of antennarotation. According to this theory, a selective filter to pass only thefundamental frequency signal of the modulating envelope and todiscriminate against the second harmonic signal produced by thecross-polarized lobes should have worked although it did not. Thus, tothe present time it has always been assumed that electromagnetic energyreceived in two adjacent cross-polarized lobes would both add to energyreceived in the principal plane of polarization in the main lobe.

In accordance with the invention, a theory was worked out, which 4as yetcannot be proved analytically, but which has been proved experimentally,lthat a fundamental frequency phase shift error could be produced bycross-polarized lobes if one added and the other subtracted from energyof the principal polarization that was received in the main lobe.Although there is still no adequate analysis which bears out thistheory, the desired result is obtained because when radiation ofcross-polarized energy is substantially prevented in accordance with theinvention, the split dish operates satisfactorily without introducingnoticeable error in the position indication of any oif-boresighttargets. It could be argued that reflected or received cross-polarizedenergy is generally only of one phase and that when this is received inone lobe it is actually added to energy received in the main lobe andwhen received in an adjacent cross-polarized lobe is subtracted from theenergy received in the main lobe; however, there is no physical basiseven to support this hypothesis. However, in spite of the misleadingpresumptions made in the prior art, the invention has proved successful.In accordance with the invention, cross-polarization is completelyeliminated and, with it, phase shift error which was produced by thesplit radar-dish of the prior art.

The invention will be better understood when considered in connectionwith the following description.

In the accompanying drawings, which are to be regarded as merelyillustrative:

FIG. 1 is a rear perspective view of the apparatus of the invention;

FIG. 2 is a schematic diagram illustrating the shape of the reliectorshown in FIG. 1;

FIG. 3 is a front perspective view of the apparatus of the inventionshown in FIG. 1;

FIG. 4 is a front elevational view of a split radardish made inaccordance with the invention;

FIG. 5 is a side elevational view of a ilat edge of one of the halves ofthe dish shown in FIG. 4;

FIG. 6 is an enlarged sectional view of half the radar reflector takenon the line 6-6 shown in FIG. 5;

FIG. 7 is a front elevational view of a radar-dish made in accordancewith an alternative embodiment of the invention;

FIG. 8 is -an enlarged view taken at area 8 shown in FIG. 7;

FIG. 9 is a sectional view taken on the line 9 9 shown in FIG. 8;

FIG. 10 is a front elevational view of still another embodiment of theinvention;

FIG. 11 is a side elevational view of one of the halves of theradar-dish shown in FIG. 10; and

FIG. 12 is an enlarged sectional view of half of the antenna systemshown in FIG. 11.

In FIG. 1, a conventional airborne radar is indicated at 10 havingspecial means 20 to open two halves 21 and 22 of dish-shaped reflector23 to a position shown at 23 in FIG. 2. Halves 21 and 22 may be hingedas indicated at 48. In the position shown in FIG. l, halves 21 and 22are disposed at an angle indicated at A in FIG. 2, the positions ofhalves 21 yand 22 being indicated at 21 and 22. The means 20 to openhalves 21 and 22 may simply include projections 25 and 26 having `cables27 and 28 iixed to them, adapted to be pulled together by any motivepower means or mechanical connection therewith. Halves 21 and 22 mayeven be opened manually since halves 21 and 22 will not be moved withrespect to each other during tracking or searching but only at aswitching time when it is desired to change from a searching to atracking operation, or vice Versa. When no force is applied to cables 27and 28, halves 21 and 22 may be kept normally closed by a spring 29.Dish 23 is illuminated by a feed 30 shown in FIG. 3, which may beconventional, to direct electromagnetic energy toward halves 21 and 22,having a plane of polarization through the axis of feed 30 extendingsymmetrically through the split between halves 21 and 22. The frontelevational view of FIG. 4 shows halves 21 and 22 when they are closedfor the searching operation and ar- 4 row 49 indicates the direction ofthe E-plane while arrow 50 indicates the direction of the H-plane.

As can be seen in FIGS. 5 and 6, each half 21 and 22 of reflector 23 isparabolic in shape having an inner lamination 31 and two outerlaminations 32 and 33 all of which may be made of any convenientdielectric fabric material. A plurality of conductors 34 are embedded ina dielectric material 35 to maintain a fixed spaced position. It is tobe noted that conductors 34 conform t0 the parabolic shape of reilectorhalves 21 and 22 but they are aligned in parallel planes substantiallyparallel to the plane of polarization of energy directed toward halves21 and 22 by feed 30.

Another embodiment of the invention is shown in FIGS. 7, 8 and 9 whereconductive parabolic halves 36 and 37 are made of a sheet metal materialhaving elongated slots 38 therein best illustrated in FIG. 8. It is tobe noted that slots 38 are ranged in two sets of rows, the first setbeing shifted lengthwise from the second set a distance equal toone-half the length of slots 38 plus one-half the length of the distancebetween slots. This means that cross-polarized energy tending to causecul'- rents to ilow in the direction of arrow 39 will be interrupted bythe slots, whereas current flowing in planes parallel to the directionof polarization and thereby parallel to a symmetrical plane through thecenter of the reflector halves 36 and 37 indicated at a split 40 willnot be interrupted because current induced by such radiation may flowcontinuously in the direction of arrow 41 between the rows of slots inan uninterrupted fashion.

As shown in FIGS. 10, 1l and 12, a parabolic solid conductive reilector41 may also be employed when split on the line 42, and semi-.circulardielectric sheets 43 may be mounted by suitable means 44 on halves 45and 46 of reflector 41. Conductors 47 may be embedded in the material ofa dielectric sheet 43 as shown in FIG. 12.

It is to be noted that conductors 47 are perpendicular to line 42whereas this is not the case in the other embodiments of the invention.This is true for the reason that reflected cross-polarized energy isagain freilected back away from reflector 41 by conductors 47 whereas,for example, conductors 34 act as a sieve and crosspolarized energysimply passes through it.

Thus, by use of the conductors of FIG. 5, the slots 38 and reflectorhalves 36 and 37 shown in FIG. 7, 0r the use of dielectric sheet 43 withconductors 47, crosspolarization can be prevented since current flow ina `direction perpendicular to the principal plane of polarization may besubstantially eliminated by the use of parallel spaced conductors. Asstated previously, this prevents unwanted phase shift error of a radiofrequency signal modulation envelope which accompanies the use of splitreliectors such as those indicated in the drawings during nutation.

Although only a few specific embodiments of the invention have been`shown and described, it is to be understood that this invention is byno means limited thereto. Many changes and modifications will, ofcourse, suggest themselves to those skilled in the art. Thus, theinvention is not to be limited by the above disclosure, the true scopeof the invention being delned only in the appended claims.

What is claimed is:

1. An aircraft radar antenna system comprising: a generally dish-shapedreector split into halves at its center by a plane through itssymmetrical axis with each half being rotated in opposite directionsaway from the other so that the intersecting obtuse angle formed bychords in a common plane subtending the arcs of each of said reflectorhalves in said common plane is increased so as to produce an antennapattern having a main lobe broader in its -Hplane than a perfectparabolic reflector of the same size; and means associated with saidreflector to discriminate against electromagnetic energy received bysaid antenna system having a predetermined plane of polarizationperpendicular to a plane through said split and through the symmetricalaxis of said reflector.

2. An aircraft radar antenna system comprising: a dish-shaped reflectorsplit at the center to provide two halves, each of said halves beingrotated in opposite directions away from the other so that theintersecting obtuse angle formed by chords in a common plane subtendingthe arcs of each of said reflector halves in said common plane isincreased, said reflector having conductor means extending substantiallyonly in one direction parallel to the direction of the split of saidreflector.

3. An aircraft radar antenna system :comprising: a dish-shaped reflectorsplit in the center for providing two halves, each half having itsrespective split edge disposed away from the split edge of the otherhalt` at an angle located in a predetermined plane perpendicular to thedirection of said split so that the intersecting obtuse angle formed bychords in a common plane subtending the arcs of each of said reflectorhalves in said common plane is increased so as to produce an antennapattern having a main lobe broader in said perpendicular plane than aperfect parabolic reflector of a similar size, said reflector havingconductor means extending substantially only in one direction parallelIto the direction of the split of said reflector.

4. An aircraft radar antenna system comprising: first and secondreflector halves, each of said reflector halves having a flat edge andbeing in the shape of one-half of a parabola of revolution about itsaxis of symmetry, each said reflector half having its respective flatedge disposed at an angle away from the flat edge of the other saidreflector half in a predetermined plane passing through said axes ofsymmetry so that the intersecting obtuse angle formed by chords in acommon plane subtending the arcs of each of said reflector halves insaid common plane is greater than if said halves were in a perfectparabolic position, said reflector halves comprising a dielectricmaterial and a plurality of spaced linear conductors having shapessimilar to the intersection with a parabola of revolution of planesparallel to said plane of symmetry.

5. An aircraft radar antenna system comprising: first and secondreflector halves, each of said reflector halves having a flat edge andbeing in the shape of one-half of a parabola of revolution about itsaxis of symmetry, each said reflector half having its respective llatedge disposed at an angle away from the flat edge of the other saidreflector half in a predetermined plane passing through said axes ofsymmetry so that the intersecting obtuse langle formed by chords in acommon plane subtending the arcs of each of said reflector halves insaid common plane is greater than if said halves were in a perfectparabolic position, said reflectors being made entirely of a oonductivematerial with a plurality of first :and second alternate rows or'elongated slots therethrough, the axis of each of said rows of slotsextending in parallel planes parallel to said plane of symmetry, saidfirst rows being spaced lengthwise from said second rows a distanceapproximately equal to one-half of the length of the slot plus oneh-alfthe distance between slots.

6. An aircraft radar antenna system comprising: first and secondreflector halves, Ieach of said reflector halves having a flat edge andbeing in the shape of one-half of a parabola of revolution about itsyaxis of symmetry, each said reflector half having its respective llatedge disposed at yan angle away from the llat edge 4of the other saidreflector half in a predetermined plane passing through said axes ofsymmetry so that the intersecting obtuse angle formed by chords in acommon plane subtending the arcs of each of said reflector halves insaid common plane is greater than if said halves were in a perfectparabolic position, said rellectors being made entirely of a conductivematerial with a dielectric supporting means affixed to the peripheraledge of said reflector so as to cover it, and a plurality of linearspaced conductors lixed 6 to said dielectric support means in parallelplanes perpendicular to said plane of reference.

7. A radar antenna system comprising: a reflector including twoindividual substantially identical halves, said rellector having agenerally parabolic shape when said halves are maintained in a firstposition contiguous to each other, each of said reflector halves havingcoincident symmetrical axes 'and a predetermined plane of separationwhen maintained in said first position, said reflector halves beingrot-atable from said first pois-tion to a second position such that eachof Isaid reflector halves is rotated at an angle in a direction oppositeto and away from the other so that the intersecting obtuse angle `formedby chords in -a common plane subtending the -arcs of each of saidreflector halves in said common plane is increased, said rellectorhalves being rotatable about an axis perpendicular to said coincidentsymmetrical axes, said rotatable axis being in said plane of separation;and means associated with said reflector to discriminate againstelectromagnetic wave energy having a plane of polarization perpendicularto said plane of separation.

8. A radar lantenna system comprising: a rellector including twoindividual substantially identical halves, said reflector h-aving agenerally parabolic shape when said halves `are maintained in a lirstposition contiguous to each other, each of said reflector halves havingcoincident symmetrical axes and a predetermined plane off separationwhen maintained in said first position, said reflector halves beingrotatable from said first position to a second position such that eachof said reflector halves is rotated at an an-gle in ra `directionopposite to and away from the other so that the chords in a common planesubtending the arcs of each of said rellector halves in said commonplane is increased, said reflector halves being rotatable about an axisperpendicular to said coincident symmetrical axes, said rotatable axisbeing in said plane of separation, said rellector halves comprises adielectric material and a plurality of spaced linear conductors havingshapes similar to the intersection of planes parallel to said plane ofseparation and a parabola of revolution.

9. A rad-ar antenna system comprising: a reflector including twoindividual substantially identical halves, said reflector having Iagenerally parabolic shape when said halves are maintained in a lirstposition contiguous to each other, each of said reflector halves havingcoincident symmetrical axes and a predetermined plane of separation whenmaintained in said first position, said reflector halves being rotatablefrom said first position toa second position such that each of saidreflector halves is rotated at an angle in a direction opposite to and-avvay vfrom the other so that the intersecting obtuse angle formed bychords in a common plane subtending the arcs of each of said reflectorhalves in said common plane is` increased, said reflector halves beingrotatable about an axis perpendicular to said coincident symmetricalaxes, said rotatable 4axis being in said pla-ne of separation, saidreflector halves being made entirely of a conductive material with aplurality of first and second alternate rows of elongated slotstherethrough, the axes of said rows of slots extending in planesparallel Ito said plane fof separation when said reflector halves aremaintained in said first position, said first rows being spacedlengthwise from said second rows a distance approximately equal toone-half the length of a slot plus one-half the longitudinal distancebetween slots.

l0. A radar antenna system comprising: a reflector including twoindivi-dual substantially identical halves, said rellect-or having agenerally parabolic shape when said halves are maintained in a firstposition contiguous to each other, each of said reflector halves havingcoincident symmetrical axes and a predetermined plane of sepaf 7 8 theother so that the intersecting obtuse angle formed in parallel planesperpendicular to said plane of separaby chords in a common planesubtending the arcs of each tion. of said reector halves in said commonplane is increased, References Cited in the fue of this patent saidreflector halves -being rotatable about `an taxis perpendicular to saidcoincident symmetrical axes, said rotatable 5 UNITED STATES PATENTS axisbeing in said plane of separation, s'aid reflector halves 2,408,373 ChuOct. 1, 1946 being made entirely of =a conductive material, -adielectric 2,522,562 Blitz Sept. 19, 1950 supporting means fixed ftosaid reflector halves in 'a po'si- 2,597,339 Krutter May 20, 1952 tionto cover said ree'ctor halves, anda plurality of linear 2,790,169 SichakApr. 23, 1957 spaced conductors fixed to said ydielectric support means10 2,870,440 Butler Jan. 20, 1959

1. AN AIRCRAFT RADAR ANTENNA SYSTEM COMPRISING: A GENERALLY DISH-SHAPEDREFLECTOR SPLIT INTO HALVES AT ITS CENTER BY A PLANE THROUGH ITSSYMMETRICAL AXIS WITH EACH HALF BEING ROTATED IN OPPOSITE DIRECTIONSAWAY FROM THE OTHER SO THAT THE INTERSECTING OBTUSE ANGLE FORMED BYCHORDS IN A COMMON PLANE SUBTENDING THE ARCS OF EACH OF SAID REFLECTORHALVES IN SAID COMMON PLANE IS INCREASED SO AS TO PRODUCE AN ANTENNAPATTERN HAVING A MAIN LOBE BROADER IN ITS H-PLANE THAN A PERFECTPARABOLIC REFLECTOR OF THE SAME SIZE; AND MEANS ASSOCIATED WITH SAIDREFLECTOR TO DISCRIMINATE AGAINST ELECTROMAGNETIC ENERGY RECEIVED BYSAID ANTENNA SYSTEM HAVING A PREDETERMINED PLANE OF POLARIZATIONPERPENDICULAR TO A PLANE THROUGH SAID SPLIT AND THROUGH THE SYMMETRICALAXIS OF SAID REFLECTOR.